Catalyst for the copolymerization of epoxides with CO2
This is a continuation of application Ser. No. 07/362,894, filed Jun. 7, 1989, now abandoned. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The invention described in this Application relates to an improved catalyst and method for making said catalyst for use in the synthesis of poly(alkylene carbonate) polymers formed by the copolymerization of carbon dioxide and epoxides. 
     BACKGROUND OF THE INVENTION 
     The copolymerization of carbon dioxide with epoxides to form poly(alkylene carbonate) polymers was discovered by Inoue and co-workers (Polymer Letters 7, 287(1969)) and described in U.S. Pat. No. 3,585,168. Other processes are described in U.S. Pat. Nos. 3,900,424, 3,953,383 and 3,248,415. Despite the obvious economic advantage associated with the use of an abundant, low cost material like carbon dioxide, the introduction of commercial products that utilize this chemistry has been very slow. One reason has been the practical difficulty in handling the organometallic catalysts required on a commercial scale. 
     The Inoue catalyst system was prepared by the reaction of diethylzinc with materials containing active hydrogen compounds., e.g., water, dicarboxylic acids, or dihydric phenols. Typical catalyst productivities ranged from 2.0 to 10.0 grams of polymer per gram of catalyst used, with more results falling at the low end of the range. Long polymerization times--24 to 48 hours--were required in order to achieve satisfactory yields and product molecular weight. Some of the phenol-containing catalysts form highly colored material on exposure to air. This can lead to discolored products because of the difficulty in removing all traces of color bodies from the polymer. A high ratio of catalyst to polymer was required to achieve satisfactory yields; therefore, separation of the catalyst from the polymer at the conclusion of polymerization was difficult. The Inoue catalysts also generated noticeable amounts of cyclic carbonate by-products and polyether homopolymer that had to be removed from the desired polycarbonate polymer. 
     Diethylzinc is a highly reactive, pyrophoric material. Stringent handling requirements are necessary to prevent the risk of fire when the material is to be used on a commercial scale. When the polymerization reaction is finished, extensive purification procedures are necessary to remove all traces of catalyst from the polymer. Because of its general lack of stability, sensitivity to moisture and to other catalyst poisons, the active catalyst material is prepared from diethylzinc immediately prior to polymerization. This is a distinct disadvantage since catalyst quality cannot be determined prior to polymerization. These considerations, along with the generally low catalyst productivity, have stimulated the search for improved catalyst materials for epoxide/CO 2  polymerizations. 
     Zinc carboxylates have also been described as effective catalysts for CO 2  polymerization. Since these are stable materials with none of the handling problems associated with diethylzinc, they represent interesting candidates for a practical commercial catalyst system. 
     Soga and co-workers reported that reaction products of zinc hydroxide and aliphatic dicarboxylic acids exhibited high activity for the copolymerization of carbon dioxide and propylene oxide (Polymer J. 13(4), 407(1981)). A variety of acids were tested, but only adipic and glutaric acid produced catalysts with higher activity than the known diethylzinc catalysts. Catalysts prepared from aromatic dicarboxylic acids were essentially inert under the Soga polymerization conditions. 
     Soga reported another approach to improve catalyst activity that involved supporting the catalyst on an inert OXide Carrier (Nippon Kagakkaishi 2, 295(1982)). Using a support increased the surface area of active catalyst material and was expected to lead to more efficient production of polymer. However, the supported catalysts were no more effective than the standard diethylzinc based catalysts. 
     The metal salts of acetic acid are a third type of catalyst material known to bring about copolymerization of CO 2  with epoxides (Soga. et al., Makromol. Chem. 178, 893(1977)). Only zinc and cobalt produced alternating copolymers from CO 2  and epoxides, and the activity of these catalysts was lower than that observed with diethylzinc derived catalysts. 
     The synthetic methods described by Inoue, particularly the handling of the organometallic catalyst and its removal from the polymer, proved to be impractical for the production of sufficient polymer for industrial testing and evaluation., therefore, an improved method of polymer preparation was sought. Of all the catalyst systems reported in the literature up to that time, only zinc carboxylates based on adipic or glutaric acid appeared to have the potential for practical use on a commercial scale. 
     Soga et al reported a synthetic procedure for production of a zinc carboxylate catalyst involving reaction of zinc hydroxide with glutaric acid in alcoholic solvents. The catalysts so obtained experimentally were inconsistent in their polymerization behavior. Some preparations gave products with good productivity while other, seemingly identical, preparations were inactive or had low productivity. In the course of investigating the causes of the inconsistent behavior, an improved method of catalyst preparation was developed. A description of the improved method and the preferred embodiments is described in the next section. 
     It has also been reported that soluble zinc catalysts can be prepared by the reaction of zinc oxide or zinc salts with a dicarboxylic acid anhydride or monoester in a suitable solvent such as the lower alcohols, ketones, esters and ethers as disclosed by Sun in U.S. Pat. No. 4,783,445. 
     In the prior art methods, preparation of the catalysts involved repeated washing and purification steps to produce an active catalyst. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention provides an improved procedure for the preparation of zinc carboxylate catalysts used in the preparation of copolymers of epoxide materials and carbon dioxide. Specifically, substantially improved zinc carboxylate catalysts for the copolymerization of epoxides and carbon dioxides are produced by reacting zinc oxide with a dicarboxylic acid. preferably glutaric or adipic acids, in a reaction medium comprising an aprotic solvent, preferably a solvent that forms an azeotrope with or is immiscible with water. The solid catalyst may then be separated from the reaction mixture and dried. Alternatively, the water of reaction may be removed by azeotropic distillation with the reaction solvent, and the resulting catalyst slurry may be used directly. The method used to prepare the zinc carboxylate has a dramatic effect on its performance as a catalyst, even though the empirical composition and the spectral properties of the carboxylates prepared by different methods are virtually identical. The present method produces a catalyst substantially free of residual water or other active hydrogen sources which are deleterious to its activity in the copolymerization of carbon dioxide and epoxides, but without the necessity of expensive and time-consuming repeated washing and purification steps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 show the effect of temperature on the catalyst. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The zinc carboxylate catalysts of this invention are made by reacting reagent grade zinc oxide, in a slight molar excess, with a dicarboxylc acid, preferably either glutaric or adipic acid, in the presence of an aprotic solvent, that is, a solvent without active hydrogen atoms. A suitable test for active hydrogen atoms is the Zerewitinoff determination Os described in standard organic chemistry texts. Preferably, the solvent forms an azeotrope with or is immiscible with water formed by the reaction of the zinc oxide and dicarboxylic acid to facilitate the separation of the water produced by the zinc oxide/acid reaction from the insoluble catalyst. The reaction can be carried out from about room temperature (20° C.) to about 85° C. and preferably up to about 55° C. The reaction, depending on dicarboxylic acid used, takes about one to about four hours at 55° C. The fine zinc carboxylate catalyst powder formed under the reaction conditions can be substantially separated from the water in the reaction medium by azeotropic distillation of the solvent/water mixture or separated by filtration from the liquid phase and then dried. 
     The product is a fine, white powder zinc carboxylate ready for use in the copolymerization of epoxides and carbon dioxide as generally taught in prior art processes utilizing other catalysts including, but not limited to, those processes described in U.S. Pat. Nos. 3,585,168 and 3,953,383. 
     A more detailed description of the invention and its methods of practice are described in the following examples. 
     EXAMPLE 1 
     Prior Art Catalyst Preparation Method 
     To a 300 ml glass reactor containing 150 ml of solvent were added 50 mmol of a dicarboxylic acid and 50 mmol of Zn(OH) 2 .The mixture was allowed to stir for four hours at 55° C. The solid product was filtered off, washed repeatedly with the solvent and dried in vacuo at room temperature. 
     EXAMPLE 2 
     Catalyst Preparation Method A 
     To a one liter, three neck, round bottom flask equipped with stirrer, condenser, Dean-Stark trap and heating bath was added 0.50 moles zinc oxide (Fisher reagent grade) and 350 ml reaction solvent. To this was added 0.45 moles of dicarboxylic acid dissolved or slurried in the reaction solvent. The mixture was heated for 4 hours at 55° C. with vigorous stirring. On cooling, the solids were filtered off and dried overnight in a vacuum oven at 80° C. 
     EXAMPLE 3 
     Catalyst Preparation Method B 
     To a one liter, three neck, round bottom flask equipped with stirrer, condenser, Dean-Stark trap and heating bath was added 0.50 moles zinc oxide (Fisher reagent grade) and 350 ml reaction solvent. To this was added 0.45 moles of dicarboxylic acid dissolved or slurried in the reaction solvent. The mixture was heated for 2 hours at 55° C. with vigorous stirring. The mixture was then brought to reflux and held there until azeotropic removal of water ceased. On cooling, the solids were filtered off and dried overnight in a vacuum oven at 80° C. 
     EXAMPLE 4 
     Effect of Dicarboxylic Acid Reactant 
     To a one liter, three neck, round bottom flask equipped with stirrer, condenser and heating bath was added 50.0 gm (0.61 moles) zinc oxide (Fisher reagent grade) and 350 ml reagent grade ethanol. To this was added a solution of 76.0 gm (0.57 moles) of dicarboxylic acid dissolved in 400 ml of ethanol. The slurry was heated for 4 hours at 55° C. with vigorous stirring. On cooling, the solids were filtered off and dried overnight in a vacuum oven at 80° C. The yield was quantitative. In a similar fashion, the catalysts described in Table II were prepared and screened using the polymerization method described above. 
     EXAMPLE 5 
     Catalyst Productivity Screening Method 
     Three to five grams of zinc carboxylate catalyst prepared according to the procedure described in Method A or B was placed in the bottom section of a 300 ml capacity Parr stirred autoclave reactor. The reactor body and catalyst were heated in a nitrogen purged oven at 125° C. for 8-10 hours, then cooled to room temperature while maintaining the nitrogen atmosphere. The autoclave body was transferred to a gloved dry box where 150 gm of methylene chloride and 50 gm of propylene oxide were added. Both materials were reagent grade chemicals, and had been dried and stored over 4A molecular sieves. The autoclave head was attached to the body and the entire assembly was transferred to a pressure cell. The reactor was connected to nitrogen and CO 2  sources via a three-way valve. The autoclave was pressurized to 300 psi with N 2 , slowly vented to atmospheric pressure and repressurized to 250 psi with CO 2 . The reactor was then heated to an internal temperature of 85° C. The CO 2  pressure increased to 450 psi and was maintained at this level throughout the course of the polymerization via a constant pressure feed valve. After 4 hours, the reactor was cooled to room temperature, the pressure was released and the contents of the reactor were emptied into a crystallizing dish. The solvent and unreacted propylene oxide were allowed to evaporate under ambient conditions for 12 hours, then the residue was dried under vacuum at room temperature for an additional 12 hours. The white, leathery polymeric material obtained was weighed to determine gross yield. After correcting the gross weight for the amount of catalyst charged, the catalyst efficiency was reported as grams of polymer produced per gram of catalyst used. 
     Further analysis of the crude and purified product by FTIR and NMR analysis showed the polymer to have substantially a 1:1 alternating structure as reported by Inoue and others. The zinc carboxylate catalysts studied in this work produced negligible amounts of methanol soluble polyether polymer. According to literature reports, the latter materials were usually present as by-products when diethylzinc based catalysts were used. The crude product was shown to contain 3-5% of cyclic propylene carbonate by-product by FTIR. The amount of cyclic by-product formed was relatively constant in all runs using zinc carboxylate catalysts. 
     EXAMPLE 6 
     Effect of Catalyst Synthesis Reaction Temperature 
     To a one liter, three neck, round bottom flask equipped with stirrer, condenser, Dean-Stark trap and heating bath was added 50.0 gm (0.61 moles) zinc oxide (Fisher reagent grade) and 350 ml toluene. To this was added a slurry of 76.0 gm (0.57 moles) of glutaric acid in 400 ml of toluene. The slurry was heated for 4 hours at the temperature indicated with vigorous stirring. The mixture was then brought to reflux and held there until removal of water ceased. The mixture was cooled and the white solid was filtered off and dried overnight at 80° C. The yield was quantitative. 
     The above examples and the data shown in the tables demonstrate that the zinc source used for synthesis proved to have an important effect on catalyst performance as shown in Table I. The prior art method used Zn(OH) 2  as the zinc source. While freshly prepared zinc hydroxide worked reasonably well, purchased zinc hydroxide proved unacceptable because of low reactivity. ZnO was found to be an effective zinc source and a more practical alternative since purchased material had equivalent reactivity to laboratory prepared material. Zinc sources with more ionic character, such as zinc acetate and zinc carbonate, produce catalysts with lower activity than those prepared from zinc oxide, even when the same dicarboxylic acid is used. 
     
                       TABLE I                                                     
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EFFECT OF ZINC SOURCE                                                     
ON CATALYST PERFORMANCE*                                                  
          Catalyst     Reaction  Catalyst**                               
Zinc Source                                                               
          Prep. Method Solvent   Productivity                             
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Zn(OH).sub.2                                                              
          B            Toluene   0                                        
Zn(OH).sub.2                                                              
          A            Methanol  4                                        
Zn(CO.sub.3).sub.2                                                        
          B            Toluene   2                                        
Zn(CO.sub.3).sub.2                                                        
          A            Methanol  1                                        
Zn(OAc).sub.2                                                             
          B            Toluene   0                                        
Zn(OAc).sub.2                                                             
          A            Methanol  2                                        
ZnO       B            Toluene   22                                       
ZnO       A            Toluene   17                                       
ZnO       A            Methanol  8                                        
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 *Dicarboxylic acid in each case was glutaric acid.                       
 **Grams of product obtained per gram of catalyst used                    
 
    
     As shown by Table II, the most preferred dicarboxylic acids for catalyst preparation are either glutaric acid or adipic acid. 
     
                       TABLE II                                                    
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EFFECT OF ACID GROUP                                                      
ON CATALYST PERFORMANCE                                                   
          Catalyst     Reaction Catalyst**                                
Acid Used Prep. Method Solvent  Productivity                              
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Oxalic    A            Ethanol  0.1                                       
Malonic   A            Ethanol  0.0                                       
Succinic  A            Water    0.0                                       
Glutaric  A            Ethanol  7.0                                       
Adipic    A            Ethanol  7.0                                       
Maleic    A            Ethanol  0.7                                       
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 **Grams of product obtained per gram of catalyst used                    
 
    
     The ratio of dicarboxylic acid to zinc salt used plays a role in catalyst performance, and is important from an operational perspective. As reported in the case of catalysts prepared from diethylzinc/water, the highest activity is observed when the reactant mole ratio is 1.0. However the presence of small amounts of free acid is deleterious to catalyst performance. Although excess acid can be removed by thorough washing, this entails extra steps in the procedure. We have found the washing steps can be avoided by using a stoichiometric excess of zinc oxide in the synthesis. Since zinc oxide is inert in the polymerization process, it need not be separated from the catalyst. A large excess of zinc oxide is undesirable because it dilutes the amount of active catalyst present per pound of catalyst/oxide mixture. On the other hand, a sufficient excess should be used to insure complete conversion of the acid. From practical considerations, we have found a 2-5 mole percent excess of zinc oxide to be most preferable. An excess of the dicarboxylic acid is to be avoided. 
     The influence of solvent used for catalyst preparation on performance is shown in Table III. Unexpectedly superior catalyst performance is obtained when solvents that are aprotic are used as the catalyst synthesis medium. Preferred are solvents that form azeotropes with water, and contain oxygen heteroatoms. Catalyst performance is slightly lower, but still acceptable, when aprotic hydrocarbon solvents are used. Solvents that are miscible with water but contain no active hydrogen atoms (e.g., dioxane, acetone) give catalysts of intermediate activity. Water and lower alcohols that are miscible with water or that have active hydrogen atoms give catalysts of lowest activity. 
     
                       TABLE III                                                   
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EFFECT OF PREPARATION SOLVENT                                             
ON CATALYST PERFORMANCE                                                   
           Catalyst          Catalyst**                                   
Reaction   Prep.    Acid     Produc-                                      
Solvent    Method   Used     tivity  Mn × 10.sup.-3                 
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Water      A        Glutaric 3       --                                   
Methanol   A        Glutaric 8       36                                   
Ethanol    A        Glutaric 7       80                                   
i-Propanol A        Glutaric 2       --                                   
70% i-PrOH A        Glutaric 5       55                                   
Tetrahydrofuran                                                           
           A        Glutaric 6       40                                   
Dioxane    A        Glutaric 8       25                                   
Acetone    A        Glutaric 5       42                                   
Hexane     B        Glutaric 9       77                                   
Toluene    B        Glutaric 22      79                                   
Toluene    A        Glutaric 17      111                                  
Toluene    B        Adipic   19      99                                   
Methanol   A        Adipic   0       --                                   
Dibutyl Ether                                                             
           B        Glutaric 26      116                                  
Dibutyl Ether                                                             
           A        Glutaric 21      98                                   
Anisole    B        Glutaric 22      173                                  
Ethyl Benzoate                                                            
           B        Glutaric 23      111                                  
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 **Grams of product obtained per gram of catalyst used                    
 
    
     Another item affecting catalyst performance is the reaction temperature and temperature profile used for the synthesis reaction, as shown in Table IV, for reactions run under the conditions of Example 6. The most preferred range for the initial reaction temperature is from room temperature to about 55° C. If higher initial temperatures are used, for example, by heating immediately to the azeotrope temperature when high boiling azeotropic solvents are used, catalyst performance is adversely affected. On the other hand, reactions carried out below room temperature do not improve catalyst performance, but require longer times for complete conversion of reactants. 
     
                       TABLE IV                                                    
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EFFECT OF REACTION TEMPERATURE                                            
ON CATALYST PERFORMANCE                                                   
Reaction       Catalyst*                                                  
Temperature    Productivity                                               
                          Mn × 10.sup.-3                            
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25             16         126                                             
30             17         161                                             
40             16         118                                             
55             13         167                                             
85             10         130                                             
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 **Grams of product obtained per gram of catalyst used.                   
 
    
     The reaction is complete in one to three hours at 50° C. when glutaric acid is used as the acid source. Adipic acid requires longer reaction time and is more difficult to drive to complete conversion. The extent of conversion is conveniently monitored by thermogravimetric analysis. Pure zinc carboxylates decompose at around 425° to 450° C. to form zinc oxide. There is no detectable weight loss below 400° C. as shown in FIG. 1. If traces of unreacted acid or solvent remain in the product, their presence will be indicated by weight loss occurring below 400° C. as shown in FIG. 2.